In order to fuse electroscopic toner material permanently onto a support surface by heat, it is usually necessary to elevate the temperature of the toner material to a point at which the constituents of the toner materials coalesce and become tacky. This heating causes the toner to flow to some extent into the fibers or pores of the support member. Thereafter, as the toner material cools, solidification of the toner material causes the toner material to become firmly bonded to the support member.
The use of thermal energy for fixing toner images onto a support member is well known. Several approaches to thermal fusing of electroscopic toner images have been described in the prior art. These methods include providing the application of heat and pressure substantially concurrently by various means, for example, a roll pair maintained in pressure contact, a flat or curved plate member in pressure contact with a roll, and a belt member in pressure contact with a roll.
Heat may be applied by heating one or both of the rolls, plate members or belt members. The fusing of the toner particles takes place when the proper combination of heat, pressure and contact time are provided typically, in such direct contact systems, the roller surface may be dry, i.e. no application of a release agent to the surface of the roller as described, for example, in U.S. Pat. Nos. 3,498,596 and 3,666,447. Alternatively, the fuser roll surface may be wetted with a release agent such as a silicone oil as described in U.S. Pat. Nos. 3,268,351 and 3,256,002. It is also known in the art to fuse toner images by the use of a flash fusing process, for example, as disclosed in U.S. Pat. No. 3,874,892. In such a process, a flash lamp is generally pulsed on for a very short period of time. It can be appreciated that since the lamp is pulsed or flashed for short period of time, a large amount of power must be used to accomplish the fusing of the toner particles.
Another method for fusing toner images to a substrate is radiant fusing. Radiant fusing differs from flash fusing in that in radiant fusing, the radiant energy source, typically and infrared quartz lamp, are turned on during the entire fusing step rather than pulsed for a short period of time as in flash fusing. Examples of radiant fuser apparatus are shown in U.S. Pat. Nos. 3,898,424 and 3,953,709. Such prior art radiant fusers are generally made of relatively heavy metallic construction which requires the constant use of a heating element to maintain the apparatus at standby temperature. U.S. Pat. No. 3,471,683 shows a heater roll with a printed circuit heating element. However, the heater roll is relatively thick and the adhesive material not suitable for relatively high temperature operation.
Such prior art fusing systems have been effective in providing the fusing of many copies in relatively large, fast duplicating machines, in which the use of standby heating elements to maintain the machine at or near its operating temperature can be justified. However, there is a continuing need for an instant-on fuser which requires no standby power for maintaining the fuser apparatus at a temperature above the ambient. It is known to use a positive characteristic thermistor having a self-temperature controlling property as a heater for a heating roller. The roller is regulated to a prescribed temperature by a heating control temperature detection element. It is known to employ radiation absorbing materials for the fuser roll construction to effect faster warm-up time as described in U.S. Pat. No. 3,669,706. It is also disclosed in U.S. Pat. No. 4,355,225 to use an instant-on radiant fuser apparatus made of a low mass reflector thermally spaced from a housing, with the housing and the reflector together forming a conduit for the passage of cooling air therein. A low mass platen is provided which is constructed to achieve an operating temperature condition in a matter of a few seconds without the use of any standby heating device. It is also known as disclosed in U.S. Pat. No. 3,948,214 to use a cylindrical member having a first layer made of elastometic material for transporting radiant energy, a second layer for absorbing radiant energy, and a third layer covering the second layer to affect a good release characteristic on the fuser roll surface. The fuser roll layers are relatively thin and have an instant-start capability. U.S. Pat. No. 4,395,109 discloses an instant-on fuser having a core of metal or ceramic supporting a fuser roller, and including a heat insulating layer, an electrically insulating layer and a protective layer formed on the outer circumference of the core.
It is also known as described in copending patent applications, U.S. Ser. No. 893,753 filed Aug. 6, 1986 and U.S. Ser. No. 893,852 filed Aug. 6, 1986 to provide instant-on fuser apparatus having a relatively low thermal mass designed for relative ease of construction and that has a relatively high mechanical strength.
The prior art is also replete with general heating control circuits. For example, U.S. Pat. No. 4,086,466 discloses an automatic heater controller responsive to the resistance of a heater element. The control circuit contains both an AC and DC power source. The resistance of the heating element is monitored by a DC voltage sensing means, which supplies an output signal to a comparator amplifier. The comparator is connected to a control means, which adjusts the power supplied to the heating element from an AC power source. U.S. Pat. No. 4,377,739 discloses an average power apparatus used for controlling the power supplied to a fuser in an electrophotographic machine. The apparatus comprises a power monitor which measures the average power supplied to the fuser. The measured average power level is compared to a "set" power level. The apparatus further comprises a digital network which receives an input signal from the power monitor and adjusts the average power level of the fuser to match the "set" power level.
U.S. Pat. No. 3,946,200 discloses a proportional temperature controller comprising an improved bridge circuit. One arm of the bridge circuit contains a temperature sensitive resistor. The other arm of the bridge circuit comprises a network of resistors containing a setting resistor which is linearly calibrated in terms of temperature. The temperature controller further comprises an operational amplifier which controls the supply of power to a heating element based on the degree of imbalance between the two arms of the bridge circuit. U.S. Pat. No. 4,471,210 to van den Eijnden discloses a fuser heat control circuit for controlling the temperature of a rotatable drum in a photocopying machine. Disposed on the drum is a temperature-sensitive resistor which varies in resistance as a function of the drum temperature. A control circuit supplies power to the fuser heating element in accordance with the varying resistance of the temperature-sensitive resistor.
U.S. Pat. No. 4,320,284 to Dannatt discloses a heated fuser roll comprising a plurality of wafer shaped heating elements. The heating elements are formed of a semiconducting ceramic material exhibiting a positive temperature coefficient of resistance. The resistance of the semiconducting wafers increases with increasing temperature. After a short operating time the semiconducting wafers reach a state of equilibrium, wherein the power supplied and absorbed by the wafers is equal to the heat dissipated by the wafers. By using the semiconducting wafers, the fuser roll has the ability to operate as a self-regulating heat source. Other examples of self-controlling heaters are described in NASA Tech Brief, December 1974; NASA Report No. TN D-7248, May 1973; and Electronic Design, 22, Oct. 25, 1979.
One difficulty with prior art fusers is that the fusers are often controlled using an external temperature sensor. Such sensors mean additional cost to the system. Another difficulty with prior art fuser controls is that the controls do not have a short enough response time for an instant-on fuser to allow the fast warm up of the fuser. Other prior art fuser controls are relatively complex and not adapted to the environment of a xerographic machine fuser control. It is an object, therefore, of the present invention to provide a new and improved control for an instant-on fuser. It is another object of the present invention to provide a control for an instant-on fuser that is relatively simple and does not require an external temperature sensor. It is a further object of the present invention to provide a fuser control for a fuser capable of being heated from room temperature to fusing temperature in less than 10 seconds. It is another object of the present invention to provide thermal circuits with a special material for the foil heater to control an instant-on fuser. Further objects and advantages of the present invention will become apparent as the following description proceeds and the features of novelity characterizing the invention will be pointed out with particularity in the claims annexed to and forming a part of this specification.